1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (hereinafter also simply referred to as “a photoconductor”) used in printers, copiers, facsimile machines, or the like in an electrophotographic system; and in particular, to an electrophotographic photoconductor having excellent durability in repeated printings and good resistance to gases, which is achieved by improvement of an additive material. The invention also relates to a method of manufacturing such a photoconductor.
2. Description of the Related Art
An electrophotographic photoconductor generally needs functions of retaining surface charges in the dark, generating charges upon receipt of light, and transporting the charges upon the receipt of light. There are two types of photoconductors: a single layer type photoconductor in which all these functions are performed in a single layer, and a laminated layer type photoconductor consisting of function-separated layers, the layers including a layer primarily contributing to charge generation and a layer contributing to retaining surface charges in the dark and transporting charges upon receipt of light.
The image formation in electrophotography using an electrophotographic photoconductor can be carried out by the Carlson process, for example. This process includes electrification of the photoconductor in the dark, formation of latent images of characters or pictures of the original on the charged surface of the photoconductor, development of the formed latent images by toners, and transferring and fixing of the developed toner images on a carrier medium of paper or the like. After the toner image transfer, the photoconductor is recycled through elimination of remained toners and residual electric charges.
Materials that can be used in electrophotographic photoconductors includes: inorganic photoconductive materials such as selenium, selenium alloy, zinc oxide, and cadmium sulfide; and organic photoconductive substances such as poly-N-vinylcarbazole, 9,10-anthracene-diol polyester, pyrazoline, hydrazone, stylbene, butadiene, benzidine, phthalocyanine, and bisazo compounds. These materials are used by dispersing in a resin binder, or by vacuum evaporation or sublimation.
With increased printing sheets accompanying construction of office network and rapid development of electrophotographic light printers in recent years, printers of electrophotographic system have been more strongly requiring high durability, high sensitivity, and quick responsiveness. Also required is minimal image performance variation that can be caused by the effect of ozone or NOx generated in the apparatus, and the influence of operating environment (temperature and humidity).
Conventional photoconductors, however, do not meet the requirements satisfactorily and have problems as follows.
One problem is wear resistance. Recently, high speed printers have been wide spread corresponding to introduction of the tandem development procedure in printers and copiers of the types even for color printing in addition to for monochromatic printing. In color printing, in particular, high positional accuracy of the images, as well as high resolution, has been occupying an important position among required specifications. The surface of a photoconductor is worn by repeated printings due to friction with paper, rollers, and a blade. If the wear is substantial, it is difficult to print images with high resolution and high positional accuracy. Although studies for improving the wear resistance have been extensively made so far, they are not yet satisfactory.
Among gases that are generated in an apparatus and affect the photoconductor, ozone is well known. Ozone is generated by corona discharge in a charger and a roller charger. The ozone that is remained or stayed in the apparatus and to which the photoconductor is exposed, oxidizes and breaks the original structure of the organic substances composing the photoconductor. As a result, photoconductor characteristics may be remarkably deteriorated. The ozone also oxidizes nitrogen in the air generating NOx, which may modify the organic substances composing the photoconductor.
The degradation of photoconductor characteristics due to gases can be caused not only by corrosion on the surface layer but also by penetration of the gas into internal layers of the photoconductor. The outermost layer of the photoconductor may be scratched away due to friction with the parts mentioned earlier in a little or large amount. Further, harmful gas may penetrate into the photosensitive layer and destroy a structure of the organic substances in the photosensitive layer. Accordingly, preventing the harmful gases from penetrating is also one of the problems. In a color electrophotographic apparatus of tandem structure using plural photoconductors, this problem is critical because the drums positioned at different places are differently affected by the gases, generating nonuniformity in the color tone, which hardly produces satisfactory images.
Regarding the gas resistance, Japanese Unexamined Patent Publication Nos. S57-122444 and S63-18355 disclose, for the purpose of improving gas resistance, the use of antioxidants including a hindered phenol compound, a phosphorus-containing compound, a sulfur-containing compound, an amine compound, and a hindered amine compound. Japanese Unexamined Patent Publication No. 2002-268250 proposes a method using a carbonyl compound, and Japanese Unexamined Patent Publication No. 2002-287388 proposes a method using a benzoate compound or a salicylate compound. Other methods for improving the gas resistance have been proposed including: use of a specified polycarbonate resin with an additive of biphenyl (Japanese Unexamined Patent Publication No. H6-75394 ), a combination of a specified amine compound and a polyarylate resin (Japanese Unexamined Patent Publication No. 2004-199051 ), and a combination of a polyarylate resin and a compound exhibiting specified absorbance (Japanese Unexamined Patent Publication No. 2004-206109 ). These methods, however, do not provide photoconductors exhibiting sufficient gas resistance, or, while exhibiting satisfactory gas resistance, do not show improvement of wear resistance and do not attain satisfactory result on other performances including image memories and stability of electric potential in repeated printings.
Japanese Unexamined Patent Publication No. H08-272126 discloses that adverse effect to the photoconductor due to the gases generated around the charger can be suppressed by combining with a charge transport layer exhibiting a specified mobility and controlling the oxygen permeability coefficient below a predetermined value. Japanese Unexamined Patent Publication No. H11-288113 discloses that wear resistance and gas resistance are improved when moisture permeability is below a predetermined value. The methods of these references, however, cannot achieve desired effects unless employing the specified charge transport polymer materials, and thus, are imposed by the limitations of mobility and structure of the charge transport material. Therefore, the methods cannot sufficiently meet the needs for a variety of electrical performances.
Japanese Unexamined Patent Publication No. 2004-226637 discloses that a single layer type electrophotographic photoconductor with good gas resistance is obtained by the use of a specified diester compound having a melting point of at most 40° C. The additive of low melting point substance causes a so-called “bleeding” phenomenon when a photoconductor containing the substance is in contact with the cartridge or other parts of the main body of the apparatus for a long time, in which the compound escapes into the contacting parts. The bleeding may generate image faults and thus, fails to accomplish satisfactory effects.
Concerning variation of characteristics in operating environment, a first problem is deterioration of the image characteristic in a low temperature and low humidity environment. In a low temperature and low humidity environment in general, sensitivity of a photoconductor apparently decreases, which reveals degradation of image quality such as decrease of density of images and deterioration of gradation of halftone images. Image memory may become significant accompanying the degradation of sensitivity. In a printing process, the image recorded as a latent image in the first revolution of the drum is affected by variation of the potential in the second and later revolution of the drum. As a result, printing may occur in unnecessary places particularly in the case of printing of halftone images. This is the degradation of image quality by the image memories. Often observed examples in a low temperature and low humidity environment, in particular, are negative memories, in which light and shade of the images are reversed.
Image characteristics may also degrade in a high temperature and high humidity environment. In a high temperature and high humidity environment in general, mobility of charges in the photoconductor is larger than in a normal temperature and normal humidity environment, which may cause image defects including excessive increase of printed density and minute black spots in a wholly white image printing (fogging). The excessive increase of printed density increases toner consumption, and destructs minute gradation due to enlarged one dot diameter. In contrast to those seen in a low temperature and low humidity environment, observed examples in a high temperature and high humidity environment often are positive memories, in which light and shade of the image reflect the right printing images.
The performance degradation due to temperature and humidity are often caused by absorption and release of moisture in resin binder in the surface layer or in the charge generation material in the photosensitive layer. To address this problem, a variety of materials have been studied including: Japanese Unexamined Patent Publication No. H6-1186 and Japanese Unexamined Patent Publication No. H7-168381 disclose the containing of a specified compound in a charge generation layer; and Japanese Unexamined Patent Publication No. 2001-13708 discloses that a specified polycarbonate polymer is used for a charge transport material in a surface layer. Unfortunately, materials have not been found that can satisfactorily achieve various characteristics including the suppression of the effect of temperature and humidity on photoconductors.
Although diverse studies have been made on photoconductor materials, an electrophotographic photoconductor that sufficiently satisfies the required performances as described above has not been obtained.
It is therefore an object of the present invention to solve the above problems and provide an electrophotographic photoconductor that achieves improved stability of electrical performances and prevents generation of image faults such as memories, irrespective of the types of organic materials of the resin binder and charge transport material, and the variation of temperature and humidity of the operation environment.